When a Session Initiation Protocol (SIP) session is established by a SIP user agent (UA) or towards a SIP UA, a codec negotiation may take place between the calling party and the called party. The calling party and the called party agree on codec(s) to be used for the voice transport and/or video transport, assuming the call is a voice call, video call, audio streaming session or video streaming session. The codec(s) to be used during the call are described in the Session Description Protocol (SDP). SDP is included in the SIP messages that are exchanged in the control plane during call establishment. SDP may also be included in SIP messages that are exchanged in the control plane during the active phase of a call, e.g. when one of the call parties wishes to add or remove codecs to or from, respectively, the negotiated set of codecs.
The SIP messages traverse a number of signalling proxies, between the calling party and the called party. These proxies may check the SDP (codec) that is negotiated and record this information in a call record for usage off line, such as charging and/or statistics.
A SIP Application Server (SIP-AS) may generate a call record, e.g. at the end of the call. The call record may contain details about the call, such as an identification of calling party, an identification of the called party, a calling party location and a call duration. The call record may also contain the negotiated SDP.
During the session the control plane carries SIP messages, while the user plane carries media, such as voice and/or video. The control plane does not carry such media. The user plane does not carry SIP messages.